This invention relates to integrated circuits, micro-sensors, and micromachining, and more particularly to bonding of silicon or other wafers.
The need for wafer-to-wafer, for example silicon-to-silicon, bonding has been known for years. In micromachining technology several known bonding approaches can be divided into several groups depending on the bonding material and physics of the process, as shown in FIG. 1. For silicon-to-silicon metal bonding, there are at least three major technologies: eutectic bonding, soldering and solid phase (deformation) welding which is similar to the well known thermo-compression wire bonding technique.
For glass as a bonding material, three major technologies are known: low temperature glass, glass frit bonding, anodic bonding and fusion bonding, which can be considered in this group because in the process of annealing the intermediate layer is of oxide (glass) nature.
For polymer bonding materials, along with various kinds of glues, the best known are negative photoresist, polyimids, epoxies and thermoplastic materials.
Depending on a specific application, one or another bonding material and technology can be used. In any case, bonding is intended to provide a certain level of bonding strength, which can be characterized by either pull or shear force required for delamination of the bonded wafers. Bonding also should provide a certain level of hermeticity (air tightness) or permeability and some other characteristics such as level of induced stress during bonding and maximum sustainable temperature.
A major goal for bonding is minimizing the bonded area on the surface of the wafer. This goal is derived from the general strategy of microelectronics technology: minimizing area minimizes the manufacturing cost. In many cases, especially in sensor applications, the bonding area can be comparable or even larger than the active area of the sensor or microelectronic device. Therefore, the need for decreasing this bonding area in order to decrease the cost of the IC or sensor die is clear.
Consider the cross section of the bonding area as shown in FIG. 2. Two wafers 1 and 7 are bonded together by associated bonding material layers 3 and 5. Layers 2 and 6 are interface surfaces between the wafers 1, 7 and the bonding material layers 3, 5. These surface layers 2, 6 have different physical properties compared to the wafers 1, 7 and the bonding material layers 3, 5 and characterize adhesion of the bonding material to the wafer surface. One can assign different values of mechanical strength, hermeticity and permeability to these various layers. Layer 4 is the interface surface between the two bonding material layers 3, 5 and is a result of how homogenous the bonding material is on the bonding interface 4 after the bonding process. If the bonding material is deposited only on one wafer, e.g. wafer 1, then the structure is simpler and layer 4 does not exist.
The quality of bonding is characterized by two parameters: pull or shear strength or force, S; hermeticity and permeability to different substances, H. As shown in FIG. 2, the quality of the bonding depends on the properties of all the layers 1 to 7 and on the geometry of the bonding area. Usually, as in the case of silicon wafers (substrates), the strengths S1, S7 and hermeticities H1, H7 of respectively the bonding wafers 1, 7 are greater than the corresponding parameters S3, S5 and H3, H5 of the bonding material layers 3, 5. (S1 refers to the force S for layer 1, etc.). Furthermore, the strength and hermeticity properties of the bonding material layers 3, 5 are usually better than the properties of intermediate layers 2 and 6. Therefore, in most cases the following inequalities apply:
S1=S7 greater than S3=S5 greater than S2=S6,
and
H1 =H7 greater than H3=H5 greater than H2=H6.
Ideally the properties of all the layers are the same. In this case the bonded structure is monolithic as shown in FIG. 3, and the strength and hermeticity are the highest possible and determined only by the surface area (in two dimensions) of the bonding area having width w.
As mentioned before, there is an economic reason to decrease the bonding area. With any given length of the bonding stripe, this area is proportional to its width w. Therefore, minimizing width w for required strength and hermeticity of the bond is one of the goals of bonding technology. It is clear, however, that there are some physical and technological limits on width w and t, where t is the other (thickness) dimension of the bonding area.
In case of an ideal bonding material (see FIG. 3) these dimensions w, t are determined by required pull and shear force applied to the bonding joint. Consider a sensor or IC die consisting of two chips each with the dimensions 2xc3x972xc3x970.5 mm bonded together. Suppose that this bonding is intended to survive a 2000 g shock. Assuming that one of the chips will be pulled apart by the acceleration force at 2000 g, one can calculate that this force is 0.1 N. If one also assumes that the length 1 of the bonding stripe is 5 mm, then it is easy to show that the width w of the bonding stripe made from the same monolithic material might be less than 0.1 xcexcm in order to withstand breaking stress. As one can see, this is much smaller than can be achieved with present processes. Therefore, the mechanical strength of an ideal bond does not limit reducing the bonding area. In addition, the minimal limit for width w is also determined by permeability of a thin layer of silicon (the wafer material). It is known that a silicon diaphragm thinner than 10 xcexcm is not truly hermetic with respect to the helium. Therefore, this value can be considered as a physical limit for silicon wafer material if true hermeticity is required.
In a real situation (see FIG. 2) the quality of bonding is determined mostly by the properties of layers 2 and 6 from both the mechanical and hermetical points of view and in general is proportional to the width w of the bonding stripe. This general rule is in direct contradiction to the goal of reducing bonding area.
This invention is directed to decreasing bonding area on the wafer and increasing mechanical strength and hermeticity of wafer-to-wafer bonding. This is achieved by one or more of the following: microprofiling the bonding area to decrease bonding area on the surface of the die without decreasing the active bonding surface area by forming at least the microstructure on the surface, increasing the bonding force and hermeticity by providing a larger bonding surface area, increasing the bonding force due to the partial substitution for pull force of shear force, and increasing uniformity of bonding by decreasing the stress induced during bonding. Microprofiling (xe2x80x9cprofilingxe2x80x9d) refers to, prior to bonding, micromachining the surface area of the wafer(s) to be bonded to define therein microstructures, e.g. ridges, trenches, pyramids, poles, cavities, etc.
These processes and the resulting structures are accomplished in various ways, including forming a negative slope on the side walls of microprofiled trenches; forming profiled bridges; forming a matching system of profiled trenches and ridges; forming a system of profiled hooks; forming a system of electrical output leads; forming metal leads in the profiled trenches and over the ridges; forming diffusion electrical feed unders; forming implantation or diffusion paths through the bridges; and profiling the ridges (opposing the trenches) to eliminate patterning on the bonding material.
Other pertinent processes and structures include forming spacers (profiled posts, ridges, etc.) to control the final thickness of the bonding material; forming barriers (additional ridges outside the bonding area) to limit the bonding area; limiting the spread of the bonding material in the process of its deposition; preventing particle contamination of the bonding area; creating a continuous surface on the side walls of the die along the bonding line for external hermetization by deposition of PECVD glass, for example; creating a continuous capillary space between the two wafers around the bonding area for external hermetization by liquid hermetic sealant, for example spin on glass; and protecting the die contact pads during external hermetic deposition.
A common principle here is the use of the third dimensionxe2x80x94the thickness of the waferxe2x80x94to decrease bonding surface area in the plane defined by the surface of the wafer but increase total bonding surface area and therefore increase mechanical strength and hermeticity of the bond.
Various materials are used for the bonding processes in accordance with this invention. A bonding process with profiled wafer surfaces in accordance with the invention in general includes several major steps: micromachining the bonding area (on one or both wafers) to define cavities therein; depositing bonding material on one or both of the wafer surface(s); processing of the deposited bonding material (by temperature, atmosphere, chemical treatment, etc.); patterning of the bonding layer; bonding during which the material fills in the cavities in the profiled surfaces; and in some cases hermetic sealing.